Cloned DNA segments of D. melanogaster chromosomes (Dm segments) which contain the structural genes coding for many of the abundant messenger RNAs expressed in cultured Drosophila cells have been recovered (Young and Hogness, 1977). A very abundant species of mRNA is complementary to a family of related but non-identical Dm segments. The mRNA homologous region has been mapped within each of these cloned DNA segments using restriction endonucleases and its sequence appears to be well conserved. We call this repeated structural gene sequence copia because it is homologous to a particularly abundant mRNA (Young and Hogness, 1977; Young and Hogness, in preparation). An intriguing property of the copia sequence involves its repetition; copia is repeated in a dispersed fashion so that copies of the gene are found at about 25 sites scattered throughout the genome. The proposed research is focused on the further characterization of copia. The following questions will be addressed: (1) What is the relation of the transcribed and translated sequence? Is there a significant difference in the size of these two sequences, and if there is, from what position within the transcribed segment is the translated sequence derived? (2) Does the dispersed repetition of copia reflect differentiation of function? Are all copia genes expressed if the gene product is required in a particular cell type or are certain copia genes selected for expression in a fashion that reflects the differentiated state of the cell? (3) How well conserved are the transcribed and translated sequences? Do all copia genes really code for identical proteins? (4) What is the effect of the genetic deletion of a single copia gene? Will the deficiency be compensated because copia is repeated? In addition the hypothesis that copia genes are transposable elements will be tested and the arrangement of copia gene sequences in the chromosomes of other Drosophila species will be examined.